# Silicone Bonding to Metal, Plastic and PCBA > How silicone bonds to metal, plastic and PCBA: ultrasonic cleaning, plasma activation, substrate-matched primer and controlled heat-cure molding. # Silicone bonding to other materials by heat-cure molding Silicone is an inert material with low surface energy: in its raw state it does not fuse strongly with metal, plastic or PCBA boards. We use clean pre-treatment, a matched specialist primer and controlled-temperature press curing to solve delamination and cracking in dissimilar-material bonding. ## Most bond failures happen before molding Delamination, cracking and poor weathering resistance in overmolded parts look like molding problems, but the root cause usually lies in the state of the substrate beforehand: residual oil, dust and oxide on the surface, or the absence of surface activation, or a primer that does not match the substrate. Any of these leaves the interface bond weak from the start, and it only shows up under bending, temperature difference or damp heat. So we standardise this stage: **clean pre-treatment → matched specialist primer → controlled-temperature press curing**. The treatment is matched to the substrate, the operating conditions and the weathering requirement, and the fix starts at the interface. The routes below describe general process directions. Primer selection, process parameters and verification method for a specific product are confirmed after assessing substrate, structure and operating environment. The bonding interface between silicone and a metal insert: coverage strength is the combined result of substrate condition, primer matching and the curing process. Step one ## Clean pre-treatment across the whole line Every insert substrate (metal, plastic, PCBA) is cleaned and activated to a fixed standard before molding, removing the oil, dust and oxide that cause bond failures, in a closed and controllable sequence. Cleaning, activation and primer condition decide interface strength; only then does the part go into heat-cure press molding. 1. 01 Ultrasonic deep cleaning Removes oil, contamination, dust and oxide residue from the substrate surface, leaving a clean interface for activation. 2. 02 Plasma surface activation Physical modification raises the surface energy of substrate and silicone, greatly improving interface bonding and preparing for primer and curing. 3. 03 Application of the matched primer The specialist primer is selected by substrate material, instead of one additive covering every material. 4. 04 Constant-temperature baking Precise baking drives off volatiles and cures the active coating, laying the base for a high-strength heat-cure bond. The whole line runs dry and clean, leaves no residue and does not corrode precision electronic components, which suits high-end electronics, medical and outdoor products. See [surface finishing](./finishing.html) for the related surface routes. Step two ## Primer families and where each one fits No one-size-fits-all primer: the formulation is matched by substrate, molding process and operating environment, and each family answers a different requirement. Metal ### Heat-cure primer for metal For overmolding stainless steel, aluminium alloy, iron, copper and other metal inserts. The additive contains metal-affinity active ingredients, with heat, damp-heat and ageing resistance that suits high-temperature compression curing. - High bond strength after molding - Withstands temperature cycling, water immersion and long-term bending without debonding - Commonly used on industrial seals, metal overmold structural parts and outdoor wear parts Plastic ### Primer for plastics For ABS, PC, nylon, PBT and similar substrates, with a formulation adjusted for plastics that tolerate little heat and crack easily. - Active ingredients penetrate quickly and bond with the plastic surface layer - Solves the poor bonding and easy delamination between silicone and plastics - Suits room-temperature and medium-temperature curing; common on consumer electronic keys, plastic overmold protection parts and smart hardware housings PCBA ### Primer for PCBA electronics A low-corrosion, low-volatiles, insulating formulation without chemistry that attacks traces, solder joints or SMD components, balancing bond performance with electrical insulation stability. - Moisture protection, leakage protection and ageing resistance - Developed for silicone potting and overmolding of PCB boards and precision electronic modules - Fits silicone and electronics integrated products Room Temperature ### Room-temperature adhesive bonding primer A one-component room-temperature curing formulation that needs no baking, for secondary operations such as double-sided tape, labels and lamination on silicone. - Fast activation and cure at room temperature, with stable tack that resists lifting - Used on silicone feet, interior parts and label patches Medical / Food ### Medical and food-grade additives A clean, non-toxic formulation with no migration and no odour, and good biocompatibility that meets medical and food-contact safety standards. - Combines high bond strength with safe use - Suits medical silicone accessories, mother-and-baby products and food-grade overmolded parts Matching ### How the choice is made Three things decide it: what the substrate is, which molding process is used, and where the product is used. - Substrate sets the direction of the active ingredients (metal affinity, plastic penetration, electronic insulation and so on) - Molding process sets the temperature tolerance and the curing method - Operating environment sets weathering, water, media and safety requirements - All of it is settled in the plan and then verified at the sampling stage Step three ## Controlled-temperature press curing Once clean pre-treatment and primer activation are complete, molding temperature and pressure are set to suit the silicone (solid or liquid), the substrate and the part structure, and the integrated cure is completed under those controlled conditions. - Molecular-level bonding Controlled temperature and steady pressure let silicone molecules cross-link and interlock with the activated substrate layer, forming a molecular-level composite rather than physical contact. - Tight interface, no gaps The bonded face is dense after molding, with water, shock, tensile and temperature cycling performance that reduces the risk of moisture ingress at the interface. - Parameters set per project Temperature and pressure are not fixed values; they follow the silicone, the tolerance of the substrate and the structure, and become the basis for volume production once confirmed. - Fewer failures at the source Pre-treatment, primer and curing are controlled as one system, which reduces the delamination, debonding and cracking that commonly affect composite parts. Molding routes and tool systems are described under [molding and tooling](./molding.html); interface strength can be verified through peel strength, thermal shock and related tests in the [R&D and testing](./lab.html) system. FAQ ## Bonding questions Why does silicone overmolding delaminate so easily? Silicone is an inert material with low surface energy, and in its raw state it does not fuse strongly with metal, plastic or PCBA substrates. Oil, dust and oxide on the substrate surface, together with the absence of surface activation and matched primer, leave the interface bond too weak, so delamination and cracking appear under bending, temperature difference or damp heat. What treatment do inserts go through before molding? Ultrasonic deep cleaning, plasma surface activation, application of the primer matched to the substrate material, and then constant-temperature baking to cure. The whole line is dry and clean, leaves no residue and does not corrode precision electronic components. Can one primer be used for everything? It is not advisable. Substrates differ in surface characteristics, temperature tolerance and operating conditions. Metal, plastic, PCBA, room-temperature adhesive bonding and medical or food-grade applications each correspond to a different specialist primer formulation, matched to the substrate and the working conditions. How is heat-cure molding different from ordinary lamination? Ordinary lamination is physical contact held by an adhesive layer. Heat-cure molding completes an integrated cure under set temperature and pressure, so the silicone molecules cross-link and interlock with the activated layer of the substrate to form a molecular-level composite with a tight interface — water, shock, tensile and temperature cycling performance is more stable. Can the pre-treatment damage electronic components? The whole pre-treatment line is dry and environmentally clean; plasma activation and constant-temperature baking are set to the tolerance of the components. For PCBA we use a low-corrosion, low-volatiles and insulating primer formulation that balances bond performance with electrical insulation stability. ## Facing delamination or cracking? Send us the substrate type, the operating environment and the failure you see. We will first establish whether it is an interface treatment issue or a process parameter issue, and then propose a fix. Request an assessment